1. Field of the Invention
The present invention relates in general to the field of signal processing, and, more specifically, to a programmable power control system.
2. Description of the Related Art
Power control systems often utilize a switching power converter to convert alternating current (AC) voltages to direct current (DC) voltages or DC-to-DC. Switching power converters provide power factor corrected and regulated output voltages to many devices that utilize a regulated output voltage. Exemplary devices that utilize a regulated output voltage include lamps, such as light emitting diode and gas discharge type lamps, cellular telephones, computing devices, personal digital assistants, and power supplies.
FIG. 1 represents a power control system 100, which includes a switching power converter 102. Voltage source 101 supplies an alternating current (AC) input voltage VA to a full, diode bridge rectifier 103. The voltage source 101 is, for example, a public utility, and the AC voltage VA is, for example, a 60 Hz/110 V line voltage in the United States of America or a 50 Hz/220 V line voltage in Europe. The rectifier 103 rectifies the input voltage VA and supplies a rectified, time-varying, line input voltage VIN to the switching power converter.
The power control system 100 includes a PFC and output voltage controller 114 to control power factor correction and regulate an output voltage VC of switching power converter 102. The PFC and output voltage controller 114 controls an ON (i.e. conductive) and OFF (i.e. nonconductive) state of switch 108 by varying a state of pulse width modulated control signal CS0. Switching between states of switch 108 regulates the transfer of energy from the rectified line input voltage VIN through inductor 110 to capacitor 106.
The inductor current iL is proportionate to the ‘on-time’ of switch 108. The inductor current iL ramps ‘up’ when the switch 108 is ON. The inductor current iL ramps down when switch 108 is OFF and supplies current iL to recharge capacitor 106. The time period during which inductor current iL ramps down is commonly referred to as the “inductor flyback time”. During the inductor flyback time, diode 111 is forward biased. Diode 111 prevents reverse current flow into inductor 110 when switch 108 is OFF. In at least one embodiment, the switching power converter 102 operates in discontinuous current mode, i.e. the inductor current iL ramp up time plus the inductor flyback time is less than the period of the control signal CS0. When operating in continuous conduction mode, the inductor current iL ramp-up time plus the inductor flyback time equals the period of control signal CS0.
The switch 108 is an n-channel field effect transistor that conducts when the pulse width of control signal CS0 is high. Control signal CS0 is the gate voltage of switch 108. Thus, the ‘on-time’ of switch 108 is determined by the pulse width of control signal CS0. In at least one embodiment, the energy transferred to inductor 110 is proportionate to a square of the pulse width of control signal CS0.
Capacitor 106 supplies stored energy to load 112. The capacitor 106 is sufficiently large so as to maintain a substantially constant output voltage VC, as established by PFC and output voltage controller 114. The output voltage VC remains substantially constant during constant load conditions. However, as load conditions change, the output voltage VC changes. The PFC and output voltage controller 114 responds to the changes in output voltage VC and adjusts the control signal CS0 to resume a substantially constant output voltage as quickly as possible. The output voltage controller 114 includes a small capacitor 115 to filter any high frequency signals from the line input voltage VIN.
In addition to regulating the output voltage VC, PFC and output voltage controller 114 controls switch 108 to provide power factor correction for switching power converter 102. The goal of power factor correction technology is to make the switching power converter 102 appear resistive to the voltage source 101. Thus, the PFC and output voltage controller 114 attempts to control the inductor current iL so that the average inductor current iL is linearly and directly related to the line input voltage VIN. Prodić, Compensator Design and Stability Assessment for Fast Voltage Loops of Power Factor Correction Rectifiers, IEEE Transactions on Power Electronics, Vol. 22, No. 5, September 2007, pp. 1719-1729 (referred to herein as “Prodić”), describes an example of PFC and output voltage controller 114.
The values of the pulse width and duty cycle of control signal CS0 depend on two feedback signals, namely, the line input voltage VIN and the capacitor voltage/output voltage VC. PFC and output controller 114 receives two feedback signals, the line input voltage VIN and the output voltage VC, via a wide bandwidth current loop 116 and a slower voltage loop 118. The line input voltage VIN is sensed from node 120 between the diode rectifier 103 and inductor 110. The output voltage VC is sensed from node 122 between diode 111 and load 112. The current loop 116 operates at a frequency fc that is sufficient to allow the PFC and output controller 114 to respond to changes in the line input voltage VIN and cause the inductor current iL to track the line input voltage to provide power factor correction. The current loop frequency is generally set to a value between 20 kHz and 100 kHz. The voltage loop 118 operates at a much slower frequency fv, typically 10-20 Hz. By operating at 10-20 Hz, the voltage loop 118 functions as a low pass filter to filter an alternating current (AC) ripple component of the output voltage VC.
The PFC and output voltage controller 114 controls the pulse width and period of control signal CS0. PFC and output voltage controller 114 controls switching power converter 102 so that a desired amount of energy is transferred to capacitor 106. The desired amount of energy depends upon the voltage and current requirements of load 112. To regulate the amount of energy transferred and maintain a power factor correction close to one, PFC and output voltage controller 114 varies the period of control signal CS0 so that the input current iL tracks the changes in input voltage VIN and holds the output voltage VC constant. Thus, as the input voltage VIN increases, PFC and output voltage controller 114 increases the period of control signal CS0, and as the input voltage VIN decreases, PFC and output voltage controller 114 decreases the period of control signal CS0. At the same time, the pulse width of control signal CS0 is adjusted to maintain a constant duty cycle (D) of control signal CS0, and, thus, hold the output voltage VC constant. In at least one embodiment, the PFC and output voltage controller 114 updates the control signal CS0 at a frequency much greater than the frequency of input voltage VIN. The frequency of input voltage VIN is generally 50-60 Hz. The frequency fSC0 of control signal CS0 is, for example, between 25 kHz and 100 kHz. Frequencies at or above 25 kHz avoid audio frequencies, and frequencies at or below 100 kHz avoid significant switching inefficiencies while still maintaining good power factor correction, e.g. between 0.9 and 1, and an approximately constant output voltage VC.
FIG. 2 depicts a frequency spectrum graph 200 of electromagnetic interference (EMI) corresponding to switch control signal CS0. Electromagnetic interference is an electromagnetic disturbance, which can bring about a degradation in performance, a malfunction, or failure of an electronic circuit. The fundamental frequency fSC0 is the frequency at which control signal CS0 causes switch 108 to turn ON. For a particular input voltage VIN, a constant output voltage VC, and a fundamental frequency fSC0 of the control signal CS0, the EMI has a peak 202 at the fundamental frequency fCS0, a peak 204 at the second harmonic frequency 2fCS0, a peak 206 at the third harmonic frequency 3fCS0, and so on. The control signal CS0 is essentially tonal, i.e. the frequency of the control signal CS0 corresponding to the EMI of FIG. 2 is essentially constant. Any variation in the fundamental frequency fSC0 of the control signal CS0 for a particular input voltage VIN is generally relatively small, such as less than 0.5% of the fundamental frequency fSC0. Although the EMI is responsive to a tonal control signal CS0, the EMI varies between fSC0+ and fSC0. The EMI peak values at the second order and higher order harmonic frequencies decrease as the order of the harmonic frequency increases.
Because of the potential of EMI to interfere with electronic circuits, many governmental entities, such as the United States' Federal Communications Commission (FCC), regulate the amount of EMI that a circuit is allowed to generate. Thus, not only can EMI interfere with electronic circuits, EMI that exceeds regulatory standards can prevent an electronic device, such as PFC and output voltage controller 114 and switching power converter 102, from being legally manufactured and/or operated.